When printing on objects or articles such as bottles or similar containers, glasses or the like, in many cases, in order to achieve a sufficiently high level of productivity, it is necessary for the operation of applying printing ink or dye or like material, which is referred to hereinafter as printing ink, to the object to be followed by a special drying operation for drying the printing ink as otherwise, that is to say without such a special drying operation, it would take too long for the printing ink to dry to such a degree that the object could be handled without giving rise to the risk of smearing or smudging the previously applied print image. The necessity for rapid drying for the purposes of increasing productivity arises more particularly in connection for example with further transportation movement of such an object, in the course of which the surface of the object which has been provided with a print image may cane into contact with some other surfaces so that the previously applied image can be adversely affected as a result. However, it should be noted that the need for drying printing ink as quickly as possible, after a printing operation has been carried out, to such a degree that the applied printing ink adheres firmly to the object and is also not smeared or smudged upon coming into contact with other surfaces also arises when a plurality of print images are successively applied to the object, in order for example to produce a composite multi-color print image. When using a screen printing procedure, the mode of operation adopted in that situation is that a partial print image is applied for example in a given color, in a printing operation using a screen printing stencil and an associated doctor or squeegee, and thereafter, in succession, further partial print images are applied for example using different colors. When a following partial print image is applied in such a procedure, the previously applied partial print image must already be dry in order to ensure that it is not smeared or smudged in subsequent printing operations.
Practical operating situations predominantly involve using printing lacquers and printing inks which polymerize or set due to the effect of UV-rays, and become dry as a result. That drying procedure has proven to be satisfactorily viable in a practical context, in particular also for the reason that, when suitable levels of radiation intensity are chosen, which are familiar to any man skilled in the art, the drying operation can be effectively carried out within a few seconds. Drying printing inks by means of UV-radiation is employed in particular in relation to objects of plastic material such as bottles consisting of polyethylene or polyethylene terephthalate, but it is also employed in relation to objects of other materials such as glass or porcelain.
In general terms the arrangement is such that the drying assembly which has a UV-radiating device is disposed in the screen printing machine and is therefore pan thereof. Normally a screen printing machine has two or more treatment stations for the objects to be printed upon. The operation of printing on the objects is effected in one of those stations and then disposed downstream of the printing station in the object transportation direction is a drying station in which the printing ink which is applied in the previous printing station is then dried by UV-rays.
Usually the design configuration in such machines is such that each station has at least one holding means which carries the object while it is in that station. Those holding means may also be arranged displaceably between for example two adjacent stations so that they can additionally be used for stepwise transportation movement of the objects in the machine. It is however also possible to provide at least one particular transportation device for transporting the objects from one station to the respective following station. Another possible design configuration is one in which a part of the object transportation movement is effected by means of the displaceable holding means while another part of the object transportation movement is effected by a particular transportation means.
The holding means are frequently so arranged that they permit a rotary movement of the objects about their longitudinal axis or about an axis parallel thereto, during the respective treatment operation. The treatment devices are generally associated with the respective treatment station and are movable only insofar as is required for carrying out the treatment operation in that station.
The drying station or stations in known machines are also disposed stationarily, with the result that, in addition to the time that the object must spend in the treatment station for the drying treatment to be performed, there is also the time required for the object to be transported from a station which is disposed upstream of the drying station in the object transportation direction, into the drying station itself. In other words, a considerable amount of time is required overall for carrying out the drying operation. In that respect, the following point should also be noted: the per se known drying of lacquers and inks by virtue of the action of UV-rays presupposes that the lacquers and inks used harden or polymerize under the effect of UV-rays. The changes or reactions which cause such hardening or polymerization, in the ink or lacquer, require in any case a certain minimum period of time. In that respect, the situation can arise where that minimum time is longer than the time required in the printing station for applying a print image to the object therein. In such cases, the maximum capacity of the printing stations is not utilized as the residence time of the objects in the individual stations will thus depend on the duration of the drying operation, and thus the beginning of the next following printing operation is not determined by the end of the preceding printing operation, but by the duration of the drying operation for the object to which printing was applied in the preceding working cycle.
The time taken for a working cycle in a printing machine is generally composed of the transportation time for the objects and the stoppage time during which the objects are being treated in the treatment stations. In many machines, in the normal course of a working cycle, the transportation time makes up a greater proportion of the working cycle than the stoppage time.